The present invention relates to an orthogonal frequency division multiplexing (OFDM) modulating signal transmission system, particularly relates to the regeneration of a symbol synchronizing signal when a received signal is demodulated.
Recently, for modulation technique suitable for digital transmission for a mobile body and ground wave digital television broadcasting, an OFDM modulated signal transmission system attracts attention. The OFDM modulated signal transmission system is characterized in that it is strong concerning multipath phasing and ghost.
The OFDM modulated signal transmission system is a system for modulating multiple carriers arranged at an interval of the same frequency fs to digital carriers by the same symbolic frequency and transmitting an information code. FIG. 13 shows one example. The y-axis in FIG. 13 shows the power of a carrier, the x-axis shows a frequency and bandwidth Bw is 17 MHz. In the bandwidth Bw, for example, approximately 800 carriers are arranged at an interval of 20 kHz. For the digital modulation technique of a carrier, differential quadrature phase shift keying is often used, however, many-valued modulation technique such as 16 quadrature amplitude modulation (QAM) and 64 QAM can be also used.
In the OFDM modulated signal transmission system, a predetermined amount of data to be transmitted is divided into 800 pieces for example and 800 carriers Cf1, Cf2, - - - Cfn are modulated by the divided data. At this time, a transmission signal transmitted from a transmitting apparatus is a signal shown in FIG. 14. That is, the transmission signal is a signal in which a symbol A, a symbol B, - - - are repeated as shown in FIG. 14, wherein a term of the symbol B is equal to a term of the symbol A. In one symbol, 800 carriers are multiplexed so that they are kept mutually orthogonal to be an OFDM modulated signal. The predetermined amount of data to be transmitted is transmitted in the form of the OFDM modulated signal. A first symbol A (50 μsec) is composed of a guard interval GI and a data interval DI. To explain further in detail, for example, the first symbol A includes 1152 samples, the data interval DI includes 1024 samples and the guard interval GI includes 128 samples. A guard interval GA (the same as GI) is an interval in which a part GA′ of the data interval DI is copied. Therefore, GA and GA′ are configured by the same signal. The data interval DI is also called an effective symbol. When the first symbol is not required to be particularly differentiated, it is called the symbol A.
In case the transmission signal transmitted in the form of the OFDM modulated signal as described above is demodulated, a reference signal S0 showing the boundary position of a symbol of the received signal shown in FIG. 14 is required to be regenerated in a receiving apparatus.
First, referring to FIGS. 6 and 7, an example that the reference signal S0 is regenerated based upon a digital received signal S1 of a conventional type will be described. FIG. 6 are block diagrams showing the schematic configuration of a transmitting apparatus-receiving apparatus according to the OFDM modulated signal transmission mode and FIG. 7 shows the operational waveforms for explaining the operation of a receiving apparatus.
As shown in FIG. 6(a), transmission data applied to an input terminal 61 is converted to an OFDM modulated signal as described in relation to FIGS. 13 and 14 in an OFDM modulation unit 62 of a transmitting apparatus and a guard interval is added in a guard interval additional unit 63 to be a signal SY shown in FIG. 14. The frequency of the signal SY is converted in an up-converter 64 and the signal is transmitted via an antenna 65 as a high-frequency transmission signal.
Next, referring to FIG. 6(b), the receiving apparatus will be described. A received signal SY received via an antenna 66 is converted to a base band signal in a down-converter 67 of the receiving apparatus and is input to an A/D converter 68. From the A/D converter 68, a digital received signal S1 shown in FIG. 7 is acquired. The signal has the same configuration as the signal SY shown in FIG. 14. The digital received signal S1 is applied to a Fourier fast transformation (hereinafter abbreviated as FFT) calculating unit 69.
In the meantime, the digital received signal S1 is supplied to a delay unit 70 and a correlation calculating unit 71. In the delay unit 70, the digital received signal S1 is delayed by time equivalent to a data interval DI to a digital signal S2 and is applied to the correlation calculating unit 71. In the correlation calculating unit 71, correlation between the digital received signal S1 and the delayed digital received signal S2 is calculated.
As the digital signal S1 and the delayed signal S2 are the same signal as described above, correlation output S3 shown in FIG. 7 is acquired for the output of the correlation calculating unit 71. The reason why the correlation output S3 is a triangular waveform as shown in FIG. 7 is that data having length equivalent to the guard interval GI is fetched, being sequentially shifted in a direction of a time base and correlation between the digital received signal S1 and the delayed digital signal S2 is calculated in the correlation calculating unit 71.
The correlation signal S3 is output to a peak detector 72, the position of a peak is detected and a correlation peak position signal S4 is output. The correlation peak position signal S4 is output to a timing generator 73. In the timing generator 73, a reference signal S5 showing a boundary position of a symbol (equivalent to S0 in FIG. 14) is generated based upon the correlation peak position signal S4 and is applied to the FFT calculating unit 69. The reference signal S5 controls the timing of the digital received signal S1 applied to the FFT calculating unit 69 and the output is applied to a demodulating circuit 74. As a result, the digital received signal S1 is correctly demodulated based upon the reference signal S5. The demodulated digital signal is output from an output terminal 75, in case the signal processing of a picture signal for example is required, required picture signal processing is executed and an image is displayed on a monitor (not shown) and others. It need scarcely be said that an image may be also recorded except a monitor or can be transmitted to another place via a transmission line. The correlation peak position signal S4 is further applied to the A/D converter 68 via a clock signal controller 76, an integrator 77, a D/A converter 78 and a voltage controlled oscillator (VCO) 79 to execute symbol synchronous control.
A case that the OFDM modulated signal is transmitted from the transmitting apparatus to the receiving apparatus via a transmission line includes a case that the transmission signal is transmitted by a so-called direct wave (also called a main wave) directly incoming to the receiving apparatus from the transmission and a case that the transmission signal is transmitted by a so-called delayed wave (also called a reflection wave) that the transmission signal is transmitted, being reflected on various things from the transmitting apparatus depending upon a situation of the transmission line, and a transmission signal is normally propagated in a state in which a main wave and a delayed wave are mixed. This is generally called multipath propagation. In such multipath propagation, when the desired to undesired ratio (D/U) of the conventional type described above is −20 dB or less for example, the receive level of a reflection wave becomes higher than that of a main wave. Therefore, the correlation peak position of a reflection wave the receive level of which is higher than the correlation peak position of a main wave is detected. As a result, a problem that the data fetch interval (hereinafter called a FFT window position) of the FFT calculating unit 69 varies, a correct symbol position cannot be detected and the digital received signal S1 cannot be correctly demodulated occurs.
The problem of the conventional type will be described further in detail below. First, the OFDM modulated signal has the guard interval GA in which a part of the data interval DI is copied and added as described above. Hereby, in the case of delay in the guard interval even if multipaths occur and a reflection wave is received, so-called interference between symbols in which each one part of the data of the symbol A and the data of the symbol B is overlapped can be avoided. Therefore, the longer the guard interval GA is, the more resistant the signal is to a reflection wave. However, as a symbol interval is fixed, the data interval DI is shortened when the guard interval GA is extended and the transmission efficiency of data is deteriorated.
Next, symbol synchronization in multipath propagation will be examined. FIG. 8 shows operational waveforms in case the receive level of a main wave MW is higher than that of a reflection wave DW. As shown in FIG. 8, the OFDM modulated signal is received as a digital signal S1 in which a main wave MW shown by a full line and a reflection wave DW shown by a broken line are synthesized. The area of each symbol shown in FIG. 8 shows the magnitude of each receive level.
The correlation of the digital received signal S1 is calculated in the correlation calculating unit 71 as described in relation to FIG. 6(b). For the output of the correlation calculating unit 71, correlation output S3 shown in FIG. 8 is output. The correlation output S3 in this case has two correlation peaks, one of which is a waveform having a first peak in the symbol boundary position of the main wave MW of correlation output, and the other one of which is a waveform having a second peak (shown by a dotted line in FIG. 8) in the symbol boundary position of the reflection wave DW of correlation output.
When the correlation output S3 is applied to the peak detector 72, the higher peak of the two correlation peaks, that is, a correlation peak position signal S4 of the main wave MW is detected. A reference signal S5 of a symbol is generated based upon the correlation peak position signal S4 of the main wave MW in the timing generator 73 and is applied to the FFT calculating unit 69.
In the FFT calculating unit 69, an FFT window position is decided based upon the reference signal S5. That is, as shown in a signal S6 in FIG. 8, an FFT window position (shown by a diagonal line in FIG. 8) to fetch data is located in a position off the reference signal S5 by a guard interval and in addition, the size of the window is equivalent to the data interval DI (an effective symbol interval). Even if the reflection wave DW is received and two peaks emerge in the correlation output S3 as described above, no problem is particularly caused in the generation of the reference signal S5 if the first peak which is the receive level of the main wave MW is higher.
However, in case a transmitting apparatus is mounted in a mobile body, for example, in case a sport such as marathon is relayed, moving, a situation of a propagation path greatly changes and multipath propagation may occur. In such a situation, the receive level of a main wave MW and the receive level of a reflection wave DW greatly change and for a digital received signal S1, a signal in which the main wave MW and the reflection wave DW are mixed is received, however, as shown in FIG. 9, a case that the receive level of a main wave MW is lower than that of a reflection wave DW may occur. That is, the case is reverse to the case shown in FIG. 8. In this case, in a correlation signal S3, a peak of the correlation output (shown by a dotted line) of the reflection wave DW is higher than a peak of the correlation output of the main wave MW as shown in FIG. 9. Therefore, when the correlation signal S3 is applied to the peak detector 72 shown in FIG. 6(b), a correlation peak position signal S4 of the reflection wave DW shown in FIG. 9 is detected for a peak position detection signal S4 and a reference signal S5 is generated.
Therefore, when an FFT window position is decided based upon the reference signal S5, a hatched part is incorporated in the demodulating circuit 74 as data used for demodulation as shown in S6 in FIG. 9. In this case, a part (shown by half-tone dot meshing) of a symbol C is incorporated in the demodulation data of a symbol B and interference is caused between the symbol B and the symbol C. As a result, a problem that the error rate of the demodulation data increases occurs. The interference between the symbol B and the symbol C is described above, however, it need scarcely be said that interference between a symbol A and the symbol B is also similar.
To reduce the generation of interference between symbols, there is a method of shifting an FFT window position by M pieces of samples from the boundary of the symbol and giving clearance. An FFT window position shown in S6 in FIG. 9 is shifted in a direction of the symbol A by giving clearance as described above. Hereby, dangerousness that data in the symbol C is incorporated is reduced. However, a problem that the guard interval GI is shortened by clearance by giving the clearance equivalent to M pieces of samples and resistance to a reflection wave delayed long is reduced occurs. A value of M described above is determined based upon experiments.
Further, FIG. 10 shows a received signal in case a reflection wave (called a preceding wave PW) preceding a main wave MW and a delayed reflection wave DW are caused. A received digital signal S1 is a digital signal in which the main wave MW, the preceding wave PW preceding the main wave MW and the reflection wave DW are mixed as shown in FIG. 10. The correlation calculating unit 71 calculates correlation between the digital received signal S1 and a digital signal S2 delayed by an effective symbol interval. To simplify the explanation, when it is supposed that the correlation waveform of a preceding wave PW, a main wave MW and a reflection wave DW can be individually acquired, correlation output S3 shown in FIG. 10, that is, correlation waveforms S3-1, S3-2 and S3-3 are acquired. The waveform of the correlation output S3 has such a shape that three triangles are arranged and the peaks of correlation output signals S3 are substantially equal.
In such a case, when time t0 showing the peak position of a first correlation waveform S3-1 of the correlation output signals S3 shown in FIG. 10 is supposed to be a starting point of an FFT window position, no problem is caused in demodulation because only a signal in a symbol A is used for demodulation data. However, an actual correlation waveform is a waveform in which three triangles of the correlation output signals S3 are synthesized as shown by a correlation waveform S7 in FIG. 10. As a result, it becomes difficult to detect the position of the time t0. The correlation waveform S3-1 of the preceding wave PW shown in FIG. 10 is drawn like it is the received waveform of a signal having the same receive level as the main wave MW, however, in case the receive level of the preceding wave is low, the waveform of the preceding wave is buried under the correlation waveform of the main wave and the position of the time t0 cannot be specified. As described above, when the position of the time t0 cannot be precisely detected and the FFT window position tries to be determined based upon the peak of the correlation waveform, a received signal in which interference between symbols is caused is incorporated in the FFT calculating unit 69 as demodulation data and the error rate of the demodulation data increases.
There is a method of shifting an FFT window position by M pieces of samples from a boundary between symbols and giving clearance as described above to reduce the generation of interference between symbols. As the FFT window position shown in FIG. 10 is shifted in a direction of the symbol A in this method, clearance can be given to the incorporation of data. Hereby, dangerousness that data in a symbol B is incorporated in the FFT calculating unit 69 decreases. However, a problem that a guard interval is shortened by the clearance by giving the clearance equivalent to M pieces of samples and resistance to a reflection wave delayed long is reduced occurs.